Effect of Alloy Addition Method on Thermal Fatigue of Cr5-Deposited Metal

Given the same alloy composition in Cr5 deposited surfacing metal, the influence of two different methods of adding Cr and C in the flux-cored welding wire (1#-chromium carbide powder; 2#-graphite and chromium powder) was studied on the thermal fatigue (TF) performance of the deposited metal. Results show that (1) for either as-welded or tempered state, after the same number of thermal cycles, the crack length of specimen 1# was greater than specimen 2#; (2) it took about 200 more cycles for the initiation of cracking and failure cracking on specimen 2# than on specimen 1#; and (3) after the same number of cycles, the length of failure cracks of the as-welded state was greater than the tempered state. Studies of the microscopic structure, number of inclusions, and grain size of both types of specimens show that only a portion of chromium carbide added in 1# flux-cored wire was oxidized in high-temperature molten droplets or weld pool, plenty of incompletely decomposed chromium carbide grains and those generated in site lead to higher amount of inclusions in the deposited metal, compared with 2#. In addition, the boundary between inclusions and matrix was distinct, and the contents of S and P were higher in 1#; a larger proportion of residual austenite and twin structures, slower transformation of the residual austenite during tempering, and coarser primitive austenite and matrix grains were found in 1#. These findings suggest that optimization of the method of adding alloy elements might be an effective and cost-efficient way of improving the TF performance of deposited metal.